Sumatran region heats up as forests disappear

Average temperatures in the Indonesian province of Jambi have risen amid clearing of vast swaths of forest, a new study show.

Areas that have been clear-cut, mostly for oil palm plantations, can be up to 10 degrees Celsius hotter than forested areas.

The warming could make water more scarce and wildfires more common in the province.

The wholesale destruction of rainforests across parts of Indonesia’s Sumatra island to make way for cash-crop plantations has not just devastated animal and plant biodiversity in the region, but may also be driving an alarming rise in temperatures on the ground, a new study suggests.

Average temperatures in Jambi province, one of the most heavily deforested regions in Sumatra, rose by 1.05 degrees Celsius (1.89 degrees Fahrenheit) between 2000 and 2015 — and more than half that increase can be attributed to the lack of forest cover, according to the new research published in the European Geosciences Union journal Biogeosciences.

The team of researchers from the University of Göttingen in Germany used satellite data collected between 2000 and 2015 by the NASA Landsat missions and the MODIS instrument, as well as data collected on the ground, to compare average land surface temperature increases in Jambi with a site that was covered by forest during the entire period (and thus considered to be unaffected by direct land-use change).

They found the temperature of the forest site rose by just 0.45 degrees Celsius during that period, suggesting that at least 0.6 degrees Celsius of the total 1.05 degree increase was due to land-use change.

“We see that transformed land uses have a higher land surface temperature compared to forest, particularly bare land and young oil palm plantations, explaining the observed surface temperature increase in the province,” Alexander Knohl, a professor of bioclimatology and one of the research team leaders, said in an interview.

On clear-cut land, used mainly for agriculture, the temperature could be up to 10 degrees Celsius hotter than in forests, while young and mature palm oil plantations could be up to 6 degrees and 0.8 degrees Celsius warmer, respectively.

Knohl said the main reason for the higher rate of surface temperature increase outside forested areas was the reduction in evapotranspiration — the process by which water is transferred from the land to the atmosphere through evaporation from the soil and other surfaces, and through transpiration from plants. Evapotranspiration provides a cooling effect, similar to the process of sweating.

Clear-cut land and young oil palm plantations have less evapotranspiration than forests, and also less shade. As a result, the ground temperature is hotter than it would be in a forested area.

“Young palm oil plantations have fewer and smaller leaves and an open canopy, thus they transpire less water,” said Clifton Sabajo, the lead author of the study. “Also, the soil receives more solar radiation and dries out faster.”

Conversely, mature oil palm plantations, those established at least five years earlier, have a closed canopy and larger and more abundant leaves, which results in a cooler ground temperature than a less mature plantation.

Knohl said the increase in the surface temperature in Jambi was significant, and could affect plants and animals.

“Surface temperature shapes the microclimate and thus ecosystem habitats that other organisms live in,” he said. “Changing the surface temperature will pose an additional challenge for the organisms to adapt.”

The observed warming might reduce how much water is available in the region during the dry season, as well as make the area even more vulnerable to wildfires.

Diki Kurniawan, an environmental activist from the Indonesian Conservation Community (WARSI), who moved to Jambi as a child in 1985, said the higher temperatures were very real.

“In the past, it was still cool during nighttime here. But now it’s hot all day long,” Kurniawan said in an interview. “I have to use cooling fans now.”

Highland areas that previously provided a respite from the heat are not immune to the changes either, he said.

“I can even feel it in Kerinci district, which is located 700 meters above sea level,” he said.

Jambi is one of several provinces in Sumatra that has experienced severe deforestation, much of it to make way for oil palm plantations. Forest cover outside Jambi’s Berbak National Park declined from 86 percent to 25 percent between the 1970s and 2009.

Across Indonesia in general, the expansion of cash crop monocultures such oil palms, acacia (for pulp and paper), rubber, and smallholder agriculture has drastically reduced the area of primary forest in recent decades.